This invention relates to a high-strength high-Cr steel which has excellent high-temperature strength and toughness as well as improved resistance to oxidation and high temperature corrosion. The steel of this invention is suitable for use in boilers, equipment employed in the nuclear industry, and equipment employed in chemical industries in situations requiring resistance to high pressures and oxidation resistance at high temperatures.
A variety of heat-resistant steels have been used in superheater tubes and reheater tubes for boilers and in the other heat-exchange tubes and heat-resistant, pressure-resistant piping in the nuclear and chemical industries. Such heat-resistant steels have excellent high-temperature strength, a marked resistance to high-temperature corrosion and oxidation, and good toughness. In addition, they must be enconomical, yet at the same time exhibit excellent formability and weldability.
Conventional steels fur such uses include (i) austenitic stainless steels, (ii) low-alloy steels such as 21/4 Cr-1Mo steel, and (iii) 9-12 Cr system high-Cr ferritic steels. High-Cr steels are superior to low-alloy steel in respect to strength and resistance to corrosion as well as oxidation, and they are free of stress corrosion cracking, which is unavoidable in austenitic stainless steel. Furthermore, high-Cr steels have a low thermal expansion coefficient and excellent thermal fatigue resistance and are still economical.
Typical examples of high-Cr steels are 9Cr-1Mo steel (ASTM T9), modified 9Cr-1Mo steel (ASTM A213 T91), and 12Cr-1Mo steel (DIN X20CrMoWV 121). Other examples having improved strength are disclosed in Japanese Patent Publication No. 36341/1982, Japanese Published Unexamined Patent Application No. 110758/1980, No. 181849/1983, and No. 89842/1987.
Heat resistant steels which contain 9-12% by weight of Cr are disclosed in Japanese Published Unexamined Patent Application No. 211553/1984, No. 110753/1986, No. 297436/1987, No. 65059/1988, and No. 76854/1988, and Japanese Patent Publication No. 8502/1987 and No. 12304/1987. These alloys contain Mo, W, V, Nb, N or the like to improve high-temperature strength.
Recently, there have been attempts to operate boilers at higher temperatures and pressures than those of conventional boilers. Thus, steel tubes for boilers which have normally been exposed at 600.degree. C. must now be subjected to high temperature of 600.degree.-650.degree. C. However, conventional-high Cr steel does not have satisfactory high-temperature strength. When a conventional high-Cr steel is used in large-diameter pipes, the wall thickness has to be increased, resulting in thermal fatigue due to thermal cycling of start-up and shut-down.
On the other hand, such steels as the 9Cr-1Mo steel and 12Cr-1Mo steel have excellent high temperature strength but they do not have a satisfactory level of resistance to oxidation and corrosion at high temperatures of 600.degree.-650.degree. C. Thus, the highest service temperature is limited up to 625.degree. C. for conventional 9-12Cr steels. In order to further improve the resistance to oxidation as well as corrosion at high temperatures, it is conceivable to increase the content of Cr. However, when the Cr content is increased to over 13%, for example, a large amount of .delta.-ferrite is formed in a matrix phase, resulting in a marked degradation in toughness and high-temperature strength. It is also possible to suppress the formation of .delta.-ferrite by the addition of Ni. However, the content of Ni and Cr increases, resulting in a decrease in thermal conductivity and a decrease in the thermal efficiency of the heat-exchanger. Furthermore, a high-alloy steel with a high content of Ni and Cr is quite expensive and is comparable with 18-8 austenitic stainless steels from a cost viewpoint.
Thus, steels which can be used at a high temperature of 600.degree. C. or higher under pressure must have high-temperature strength superior to that of conventional high-Cr steels, and furthermore improved resistance to oxidation and corrosion at high temperatures compared with those of conventional high-Cr steels. They must also have toughness, formability, and weldability which are comparable to or superior to those of conventional steels.